SEOUL, SOUTH KOREA — North Korea vowed not to rule out a “new form” of nuclear test Sunday after the UN Security Council condemned its latest ballistic missile launch amid simmering tensions over Seoul’s joint military drills with Washington.

Pyongyang has carried out a series of rocket and short-range missile launches in recent weeks which have prompted stern reactions from South Korea and the United States.

On Wednesday it upped the ante by test-firing two mid-range ballistic missiles capable of striking Japan, sparking condemnation from the United Nations Security Council (UNSC).

“(We) would not rule out a new form of a nuclear test for bolstering up (our) nuclear deterrence,” Pyongyang’s foreign ministry said in a statement carried by the North’s state-run KCNA news agency.

The UNSC said the North’s missile launch Wednesday was a violation of UN resolutions barring Pyongyang from any nuclear or ballistic activity, agreeing to consult on an “appropriate response.”

Pyongyang slammed the UNSC criticism as “absolutely intolerable”, defending the launch as a “self-defensive” act in protest against the ongoing Seoul-Washington drills being held in South Korea.

The North has habitually lashed out at the annual Key Resolve and Foal Eagle exercises between the two allies — held this year from February to April — labeling them as practice for war.

“The UN Security Council, shutting its eyes to the US madcap nuclear exercises, ‘denounced’ (our) self-defensive rocket launching drills to cope with them as a ‘violation of resolutions’ ... It is absolutely intolerable,” said the ministry.

Pyongyang also warned the US to “stop acting rashly,” saying it was ready to take “next-stage steps which the enemy can hardly imagine.”

“If a catastrophic development which no one wants occurs on the peninsula, the US will be wholly responsible for it,” the ministry said.

South Korea expressed “serious concern” on the latest threat and warned of potential consequences if Pyongyang pushes ahead with an atomic test.

“The North should bear in mind that, if it pushes ahead with a nuclear test despite grave warnings of neighbors and the international community, it will pay the price without fail,” Seoul’s foreign ministry said in a statement.

Wednesday’s tests — believed to be the first mid-range missile launch since 2009 — coincided with a summit attended by the South, the US and Japan aimed at uniting the three nations against Pyongyang’s nuclear threat.

The impoverished but nuclear-armed state has staged three atomic tests in 2006, 2009 and last year.

Pyongyang’s powerful National Defense Commission, chaired by the North’s leader Kim Jong-Un, threatened on March 15 to demonstrate its nuclear deterrence.

But the country has shown no signs of launching an imminent atomic test, Seoul’s military said last week.

The North’s third atomic test in February 2013 — its most powerful to date — drew widespread international condemnation as well as new UN sanctions.

The North responded angrily, putting its “strategic” rocket units on a war footing and threatening to strike targets on the US mainland, Hawaii and Guam, as well as in South Korea.

'Angry protest'

But most experts believe the North is still years away from developing a genuine inter-continental ballistic missile that could strike the continental United States.

The latest threat is aimed at warning the US and the international community against yet more sanctions, said Kim Yong-Hyun, a professor of North Korean studies at Seoul’s Dongguk University.

“I see this as more of an angry protest rather than a genuine, serious threat to hold another nuclear test soon,” Kim said.

The “new form” of nuclear test could perhaps lay the stage for a test based on new uranium-enrichment technology, he said.

The North in 2010 unveiled to a visiting US scholar an apparently operational uranium-enrichment plant, which could give Pyongyang a second way to make nuclear bombs on top of its existing plutonium stockpile.

Uranium enrichment carries a far smaller footprint than plutonium. It can be carried out using centrifuge cascades in relatively small buildings that give off no heat and are less visible from satellites.

The North is believed to be capable of producing nuclear weapons from enriched uranium, Seoul’s defense chief said last November.

The six-nation disarmament talks aimed at curbing the North’s nuclear ambition have been on a standstill since the last meeting in December 2008.

Pyongyang wants to resume the aid-for-denuclearization talks — involving the two Koreas, China, the US, Russia and Japan.

But Seoul and Washington have refused to do so until the North shows “sincerity” for disarmament.